The present invention relates to fabrics formed from ultrahigh tenacity and modulus fibers, and particularly to heat-shrinkable and heat-settable fabrics formed from ultrahigh tenacity and modulus polyolefin fibers, as well as to methods of heat-shrinking and heat-setting such fabrics.
Fabrics are conventionally produced by weaving, knitting or otherwise forming shrinkable fibers such as wool, silk, cotton, polyesters, acrylics and polyamides. After forming, the fabric is heated to a temperature below the melting point of the fiber (and typically above its minimum crystallization temperature) whereat the fiber shrinks slightly (e.g. 1-10%). The shrinking relieves strains caused by the forming (e.g. weaving) process, tightens the fabric, evens the bearing load of the fibers and improves the feel of the fabric. If the heating is applied with the fabric under stress (or strain), either of a stretching or deforming (e.g. creasing) nature, the fabric will also set in the shape which it assumes under the stress (or strain).
Fibers of ultrahigh tenacity (e.g. 20 g/denier or more) and modulus (e.g. 600 g/denier or more), such as polyaramids, graphite, boron and polybenzothiazole, have been used or proposed for a variety of applications including composites, ballistics protection, sails and puncture resistant articles of clothing. In some of these applications (e.g. sails and body armor), the fiber may take the form of a fabric. The known ultrahigh tenacity and modulus fibers do not heat-shrink or heat-set, however. The utility of a high performance fiber in fabric form would be enhanced if it could be shrunk or set, while substantially retaining the fiber properties. In addition to aesthetic advantages, a heat-shrunk or heat-set fabric could exhibit superior mechanical properties by the load-equalization, even if the individual fiber properties remained unchanged or declined slightly. To achieve these benefits with polyaramid fibers, fabrics have been prepared with a shrinkable lower performance fiber as the woof yarn and the polyaramid as the warp yarn, or vice versa.